In the traditional covering information systems, each object has only one scale, that is, the single-scale covering information system. But in realistic applications, one often needs to process and analyze data in different scales. To solve this problem, a new multi-scale covering information system is first introduced in this paper, and the optimal scale combination selections based on information entropy in multi-scale coveing decision systems are discussed. Firstly, entropy consistent scale combination and entropy optimal scale combination are defined in multi-scale covering information system. Secondly, entropy optimal scale combination is defined in multi-scale covering decision system, which is proved to be equivalent to the optimal scale combination in consistent multi-scale coveing decision system. Moreover, lower approximation entropy optimal scale combination is defined in the inconsistent multi-scale covering decision system. Finally, the corresponding selection algorithms and specific examples are given by using conditional entropy and limitary conditional entropy, respectively. INDEX TERMS Covering information system, information entropy, multi-scale covering decision system, optimal scale combination.